This invention relates to methods and machines for screeding, that is, spreading, distributing, grading and smoothing and/or leveling placed and/or poured, uncured concrete or like loose, spreadable material such as sand, gravel or relatively viscous, fluid materials. More particularly, the invention concerns an apparatus and method for screeding such materials without the need for pre-positioned rails or guides, especially rail guided paving and screeding machines such as slip form pavers. The invention is an improvement of an earlier apparatus and method for screeding such materials with a device which is supported above and moved along an area of such loose or plastic material like uncured concrete.
The present invention is an improved version of the screeding apparatus and methods of U.S. Pat. Nos. 4,930,935 and 4,655,633, both of which are assigned to the assignees of the present invention. In the device and method of U.S. Pat. No. 4,930,935, a self-propelled apparatus includes a steerable, self-propelled frame, a cantilevered boom, and an auger-type, vibratory screed having a strike-off member for engaging the concrete prior to engagement by the auger while the vibratory screed smooths the concrete after engagement by the auger. The elevation of the screed is adjusted automatically by a screed control assembly relative to a laser beacon reference plane positioned off of and remote from the apparatus such that the finished height of the concrete or other material is accurately controlled within close tolerances.
During use of the vibratory screed of U.S. Pat. No. 4,930,935, it was found that with certain types of materials, and especially stiffer or partially set concrete, or large aggregate concrete, the screed assembly of U.S. Pat. No. 4,930,935 encountered difficulties in closing all voids and openings in the concrete and producing the same high quality finished surface while operating at a normal screeding speed. Specifically, with concrete which had partially setup or was held in a concrete delivery truck for too long a time, or was placed in a thinner layer such as low slump two or three inch thick layers, or included large size stone or aggregate in the mixture, the screeding apparatus of U.S. Pat. No. 4,930,935 was required to labor more and be moved over the surface of the poured concrete more slowly in order to produce the same quality finished surface. Particularly when aggregate of large size was used in such concrete, unless the screed assembly was operated at a slower rate of movement, voids in the surface of the concrete were not fully closed. Accordingly, in such situations, the square footage area of concrete which could be finished and screeded in a given work period was reduced because of such slower operating speed. Completion of projects was, thus, delayed while the expense of concrete finishing was increased.
Accordingly, the present invention was devised to improve the screeding and/or finishing of material such as poured, uncured concrete and especially stiffer concrete which is low slump, large aggregate, or partially set, by including an additional oscillating/reciprocating element to better consolidate the concrete being worked at normal screeding speeds while eliminating voids and openings, and thereby provide a smooth high quality, properly finished surface.